WebPEncode: An additional check. Start VP8EncLoop/VP8EncTokenLoop only if VP8EncStartAlpha succeeded.
WebPEncode: An additional check.
Start VP8EncLoop/VP8EncTokenLoop only if VP8EncStartAlpha succeeded.

Change-Id: Id1faca3e6def88102329ae2b4974bd4d6d4c4a7a

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